Rocket engines are used for missiles, for launching space bound vehicles, and control of spacecraft in space. Rocket engines that are used to control the attitude of a spacecraft in space are generally referred to as thrusters. Firing a thruster produces a propulsive force that is opposite to the direction of the combustion gases exiting the nozzle of the thruster. The duration of firing and the timing of firing of multiple thrusters can be adjusted to impart sufficient torque on the spacecraft to obtain the desired attitude of the spacecraft. Thus, multiple thrusters are rapidly and repeatedly fired for accurate control of spacecraft attitude.
Thrusters typically use liquid monopropellants such as hydrogen peroxide (H2O2) or hydrazine (N2H4), or room temperature storable bipropellants such as nitrogen tetroxide (N2O4) and hydrazine. Because these thrusters operate in a zero gravity environment, the propellant can float in the propellant tank resulting in pressurizing gas or propellant vapor instead of liquid propellant being fed out of the propellant tank. In current practice, one means of zero-gravity feed is to use a flexible or thin metallic diaphragm, separating the pressurizing gas from the liquid propellant. Another currently practiced means of zero-gravity feed is the use of surface tension propellant acquisition devices, which draw liquid propellant towards the tank exit through capillary action.
Nitrogen tetroxide and hydrazine are toxic and environmentally hazardous. Other propellants, such as nitrous oxide (N2O) and liquid oxygen (O2) combined with a variety of fuels do not have these drawbacks, but there is no currently developed diaphragm tank technology for liquid oxygen and surface tension devices for nitrous oxide and liquid oxygen have not yet been demonstrated. Furthermore, surface-tension devices only operate in extremely low gravity environments, and many vehicles require a propellant feed solution which works under a range of acceleration environments and directions.
Another means of zero-gravity feed is to store the propellant in the propellant tank in a self pressurized state so that no additional pressurizing gas is required (sometimes called VaPak in the prior literature). Self-pressurized propellant by itself does not provide zero-gravity feed; however it is possible to consistently feed the gaseous form of the propellant out of the propellant tank by means of a heat exchanger between the withdrawn propellant and the tank contents. However, as the propellant is fed out of the propellant tank, the pressure of the tank drops causing the liquid part of the propellant to boil. The boiling causes heat to be drawn from the propellant to further drop the pressure of the propellant in the tank. Accordingly, in order to withdraw gaseous propellant consistently, the pressure of the tank drops significantly during use, to a degree which makes it impractical to use more than a small fraction of the tank contents in this way.
Rocket engines can be radiation cooled, which means that a rocket engine can be allowed to radiate its heat to space. The use of certain propellants, such as liquid oxygen or nitrous oxide with most fuels prohibits radiation cooling because these propellants burn very hot and radiation cooling cannot provide sufficient cooling. Certain other rocket engines circulate some of the propellant in passages in the nozzle and/or the combustion chamber to cool the rocket engine, which is called regenerative cooling. The latter cooling method, however, is not applicable to small thrusters because there is not enough cooling capacity in the propellant to cool the engine.
A solution to the above problems that currently exists is to run a coolant through the passages in the nozzle and/or the combustion chamber. The problem with this type of system, however, is that continuation of the cooling cycle may excessively heat the coolant.